Field of the Invention
The present invention relates to an image forming apparatus that monitors the color tone in image formation.
Description of the Related Art
There is an image forming apparatus that reads images formed on paper sheets. In this image forming apparatus, a reading unit formed with an in-line sensor or the like is connected to the downstream side of the image forming unit that forms images on paper sheets. There also is an image forming system that reads images formed on paper sheets with a reading device (a printout reading device) connected to the latter stage of an image forming apparatus that forms images on paper sheets.
In such an image forming apparatus and an image forming system, a predetermined patch image (a correction image) is formed on a paper sheet on a regular basis, and the patch image on the paper sheet is read. Consequently, any color change (color tone difference) in the image forming apparatus can be sensed and corrected (calibrated).
However, due to this patch image formation on paper sheets, extra paper sheets for the patch image formation are consumed. Further, due to the output of the paper sheets having the patch image formed thereon, the productivity in regular image formation becomes lower.
To counter these problems, a technique has been suggested to reduce such extra paper sheet consumption and the decrease in productivity. According to this technique, image data for image formation is compared with printout read image data pixel by pixel. As this technique does not involve patch image formation on paper sheets, the extra paper sheet consumption and the decrease in productivity are supposedly reduced.
JP 2014-182353 A suggests various techniques related to this kind of technology.
For an image forming apparatus that forms toner images by an electrophotographic method, a patchless color correction technique that does not require any special chart and does not generate any spoiled paper (or minimizes spoiled paper) has been suggested to maintain color reproduction stability when a color change (a color tone difference) occurs due to various factors. According to this technique, original image data is compared with read image data of a printed material, and color reproduction is analyzed so that the result of the analysis can be used in color correction. However, if the data necessary for calibration cannot be obtained from the original image data that is normally used, or if the amount of data is too small, calibration cannot be performed.
JP 2014-182353 A also discloses a method of monitoring the color change (color tone difference) between output images on different pages or between the same pages of two or more copies. However, according to JP 2014-182353 A, the monitoring cannot be performed unless there is a flat region with a certain width. Further, only changes in the colors in the monitor target regions can be monitored, and the method is compatible only with the colors that satisfy certain conditions.